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<br />~n"5('HI <br />u d ,L' \. :".1 v <br /> <br />4.2 GEOLOGY <br /> <br />4,2,1 The project area is underlain by Quaternary Holocene alluvium- <br />colluvium, Pleistocene gravel, and Cretaceous bedrock consisting of <br />Mancos shale, Iles Fonmation, and Williams Fork Fonmatlon, Late <br />Mesozoic and Cenozloc sediments dip about four to ten degrees <br />northeasterly into the Red Wash Syncl ine, The area is within a broad <br />tlood plain bounded by steep terrace capped slopes and cl iffs, At <br />least four main terrace levels and several secondary levels, can be <br />distinguished above the valley floor, particularly in the southern and <br />eastern slopes of the valley, The north and west sides of the river <br />valley show little or no terrace development due to subsequent erosion <br />by the White River and tributary drainages, Shales of the Buck <br />Tongue, Mancos shal e and sandstones and shal es of the Mesa Verde <br />fonmatlon constitute bedrock throughout the project area and dip four <br />to ten degrees northeast, The perimeter of the project area is <br />bounded by slopes that range from vertical sandstone cliffs to nearly <br />mantled by slopewash talus and residual soils, The terrace and <br />alluvial fan deposits are up to 25-feet. C7 .om) thick, but rrost are <br />less than 15-feet (4.6m).(24) <br /> <br />4.2.2 Seismic activity has occurred In the recent past, rrost of which <br />Is presumed to have been Induced by water injection into oil wells of <br />the Rangely Oil Field, 6 miles (9.6 km) to the west of the dam sites <br />proposed in Alternatives A, B, and C. During an experiment conducted <br />by USGS, one potentially active fault has been Identified by the <br />Colorado Geological Society. This fault lies 7 mIles (11.2 km) west <br />of the dam sites proposed under Alternatives A, B, C. Seismic rIsk <br />maps by Alergmlssen and Perkins Indicate that there Is a ten percent <br />chance that a shock will occur In fifty years that wi 11 produce an <br />acceleration of over four percent of gravitational acceleration. The <br />maximum event predicted by Kirkham and Rogers in their 1979 report for <br />USGS would have a magnitude of 6.5 on the Richter scale, The maximum <br />recorded historic event, the Axial Basin Event of December 1891, Is <br />estimated to have an Intensity of VI on the Modified Mercall I scale. <br />The ep I center of th I s event I s est Imated to be 46 mi 1 es C74 km) <br />northeast of the Alternative A dam site, <br /> <br />(33) <br />37 <br /> <br />i _'" <br />